Dell U2421E connect with MacBook Pro using USB-C cable

Similar to an earlier post, I can't reliably get this brand new monitor to be detected on my 2019 Macbook Pro. I've read https://www.dell.com/support/kbdoc/ro-ro/000131273/using-a-dell-ultrasharp-usb-c-monitor-with-a-mac#Monitors_With_USB-C

Looks like it should work, in DP Alt mode. When I use the USC-C Gen2 cable that came with the monitor, the laptop charges, detects and uses the ethernet port, attached USB devices: Logitech 615 webcam and Jabra 710 speaker/mike - but doesn't detect the monitor. I've switched it out with a Thunderbolt 3 cable I have used with another Dell U3419W monitor I have, that doesn't work either. I'm running the latest monitor firmware, Monterey 12.2.1 latest update. I also have a Gen1 cable that doesn't work. I've run diagnostics on the monitor, tried display settings to scan for the monitor (that's a chicken/egg issue too - the Night Shift button doubles as a Scan for monitors button if you hold the Option key - but that only works when a monitor is detected! Luckily I had another monitor attached and working. I've also tried it with another , newer Macbook Pro.

I've spent 6-8 hours fooling with this, I've resigned to running an HDMI cable and dongle, which works alongside the USB-C...but I shouldn't have to run that other cable AFAIK.